Scanned light beams have been used for many purposes, e.g., bar code scanners, intrusion detection, touch screens, and laser light shows. In all of these applications, a light source, usually a laser, produces a beam which is scanned by some device, e.g., a rotating mirror or an oscillating mirror. In many of these applications, the beam scans across a well-defined area in a plane, and the interruption of the beam by an object crossing the plane is detected by its effect on the path of the light beam. Typically, the uninterrupted light beam strikes one or more mirrored surfaces that redirect said beam towards a photodetector, producing a steady signal level on said photodetector. Interruption of the beam produces a measurable change on said steady signal, and this change is used to trigger a signal that the beam has been interrupted.
In U.S. Pat. No. 4,762,990, a light beam is swept across an area, reflecting off of mirrors and retro-reflecting material, and the retro-reflected light is directed toward a photodetector. Interruption of the light beam by an object, e.g., a finger or a stylus, is detected by a change in the retroflection signal. The timing of the interruption signal is used to determine the position in the area where the interruption occurred, and this position then serves as an input to a data processing device. Said data processing device may he a computer and said area may be a touch screen terminal used to provide such input.
In U.S. Pat. No. 4,004,805, a pair of collimated light beams aided by a series of mirrors are projected parallel to various boundary lines of a tennis court, and an additional light beam is projected along the top of the net. Said light beams are directed towards photo-detectors, and interruption of the light signal by a tennis bail is detected to indicate that the ball is “out” or has hit the top of the net. Said electronic line monitoring system does not employ scanning nor does it provide any alternative for a traditional tennis net.
In U.S. Pat. No. 4,894,528, a pair of collimated laser beams are projected parallel to the top of a tennis net and directed towards photo-detectors. Said laser beams are positioned to be interrupted by a tennis ball if said ball hits the top of the net, thereby providing an electronic notification of a “let” serve. Said laser beam tennis net “referee” does not employ scanning nor does it provide any alternative for a traditional tennis net.
It is contemplated that the system be configured such that laser exposure does not harm the bodies, especially the eyes, of the users.
The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe, i.e. that has a negligible probability for creating damage. It is usually about 10% of the dose that has a 50% chance of creating damage under worst-case conditions. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time.
A calculation of the MPE for ocular exposure takes into account the various ways light can act upon the eye. For example, deep-ultraviolet light causes accumulating damage, even at very low powers. Infrared light with a wavelength longer than about 1400 nm is absorbed by the transparent parts of the eye before it reaches the retina, which means that the MPE for these wavelengths is higher than for visible light. In addition to the wavelength and exposure time, the MPE takes into account the spatial distribution of the light (from a laser or otherwise). Collimated laser beams of visible and near-infrared light are especially dangerous at relatively low powers because the lens focuses the light onto a tiny spot on the retina. Light sources with a smaller degree of spatial coherence than a well-collimated laser beam lead to a distribution of the light over a larger area on the retina. For such sources, the MPE is higher than for collimated laser beams. In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Although the MPE is specified as power or energy per unit surface, it is based on the power or energy that can pass through a fully open pupil (0.39 cm2) for visible and near-infrared wavelengths. This is relevant for laser beams that have a cross-section smaller than 0.39 cm2. The IEC-60825-1 and ANSI Z136.1 standards include methods of calculating MPEs.